The discovery of petroleum production fields, situated at sea, in deep and ultra-deep waters (water layers exceeding 2,000 meters in depth), calls for the use of floating structures with a capacity and strength for receiving equipment and installations necessary for its exploitation and production. These floating structures (or Stationary Production Units—SPUs) are connected to risers for production and export of the fluids produced by the wells, the former being subject to the action of sea currents.
Due to the increase in the depth of the water layer, the production capacity of the wells and the safety requirements for operating in these regions, the search for technical solutions for facing the challenges becomes increasingly complex, which can raise the costs involved hugely to the point of making a whole enterprise unviable.
One of the main challenges to be faced relates to the load to be supported by the floating structure, hence the emergence of various innovative embodiments connected with the choice of the most suitable type of structure and with the anchoring systems not only of the floating structures themselves but also of the production risers. However, most of the innovations apply to conventional structures such as semi-submersible rigs and tankers.
The principal feature of semi-submersible rigs is that they remain in a substantially stable position after being anchored and have small movements under the action of environmental forces such as, for example, wind, waves and sea currents. Nevertheless, these rigs have limited capacity for receiving loads, which impedes their use for installing equipment used in processing plants or for storing large quantities of oil. Another factor to be considered is the complexity of constructing their hulls, which requires a construction technique which is different, specialized and not very flexible regarding design changes, with reflex actions which significantly burden the design work.
On the other hand, the use of tankers, adapted for receiving production plants on their decks has been a fairly well used alternative, as these ships have large storage capacities. However, the question of load distribution in terms of the place for installation of the equipment and the structure of the tanker form one of the main problems for this type of structure, which has to be very well evaluated.
More recently, deep-draught structures of the mono-column type, known by persons skilled in the art as SPAR type platforms, have been proposed. These enormous platforms include, in general, a large cylinder supporting the installations and equipment; the cylinder is moored to the sea floor with cables and lines. This type of structure is designed to to undergo few movements and to be used in deep waters. However, it has the drawback of having a draught which is very great in length and a small surface area for accommodating installations, which makes it difficult to fit the installations normally necessary on its deck and to transport it to the oil production site. It therefore requires installation of the deck to be carried is out on the open sea, burdening the project due to the need for various different sea support auxiliary vessels to be used, these being highly sophisticated and of high operational cost.